This is Funimation we're talking about; they'll do it as soon as they possibly can.
Right now, no one wants to buy it because everyone either: Already owns DBZ(Probably in multiple forms), prefers Kai, or is holding out hope for Toei to do something in HD now that the broadcast audio for Z is out there.
But, as soon as there's a reason for them to do a release(Off the top of my head: Toei does a new remaster based on the original negatives, 4K or 8K become standard, enough buzz is generated for a reprint of the Dragon Boxes, or hell, maybe the sky falls, pigs fly, and Funimation somehow negotiate to do a DVD release of the Canadian dubs), they'll do one. That's how business works.
Here's the thing, I wouldn't discount the possibility of Funimation doing a new release based on a new thing from Toei; in Japan, there's only one release of anything Dragon Ball from before Kai,* which is the Dragon Box DVDs. It's one consistent DVD release of all three original series, all seventeen original movies, and all three TV specials. In Japan, if you want Dragon Ball, you buy the Dragon Boxes. That's it.
However, this means the shows are only available in standard def, GT uses audio from the crappy optical tracks on the film, rather than the
still existent D2 master tapes, and in the GT TV special, despite the audio being the clean, crystal-clear master audio, this is because they also took the video from the D2 master tapes, which are blurry, incorrectly telecine'd(24p > 30i frames per second conversion. In theory, this is a very simple process, but Toei screwed it up, and it doesn't convert back to 24p properly), and faded. Even at standard def, it looks terrible.
*
Of course, I'm referring to the anime here. Although, the manga was in a similar situation: If you just wanted to read the manga, you'd buy the tankobon volumes. If you didn't mind spending a little extra, and you wanted it to have colour, and its revised ending, you bought the Kanzenban volumes. Recently, they did the full colour volumes, and those sold pretty well. So I suppose the manga has already had its equivalent of this hypothetical HD release.
In theory, a new HD release would be a perfect chance to fix this stuff, along with many of the other issues in the Dragon Boxes(Such as the
slightly off framing and weird colour issues in the DB boxes, random episodes having
miscellaneous unexplained issues plaguing them, etc.), while also meaning they'd have a scan of the film elements preserved(Those films aren't getting any younger), and Japan would get a HD release of the franchise. Plus, since the shows are old anyway, keeping them in 4:3 is a no-brainer(Boo Kai was only widescreen because, as a new TV show, it was expected to air in widescreen, and it meant they didn't have to clean the tape marks, which -- as with most changes in the Boo arc of Kai's remastering -- made it cheaper; DB, Z, and GT currently air on TV in 4:3 anyway, and actually have a market in Japan, so 4:3 is the way to go for them), plus Toei could contact Kei about using his DB and Z broadcast audio, and in theory, Funimation might then import the masters from this theoretical HD release for a future release.
Of course, this is just an idle fantasy at the moment, but you have to admit -- it's an exciting possibility.
The point of Dragon Ball is to enjoy it. Never lose sight of that.
I always say it's always a WHEN not an IF when it comes to dbz getting rereleased. 
I want to say too that I wont be buying it as I have purchased everything under the sun so I'm just going to keep my mouth shut. 
I'm not going to completely bash Funi though like I know a lot of people like to do for releasing so many different versions. 
